I built a notification system on my website. I've some kind of groups of members. When a member of a group posts something, the other members of the same group should be notified. Notifications appear when the user refreshes the page. However, the world is going in real-time. Googleing the issue recommended me Node.js (Not sure is the best way). I can now: create a server, connect to a database, querying the database, broad messages using socket.io to all connected clients...
But what I can't do/understand so far is to specify to which user(s) the message(notification) should be delivered.
Looking around SO comes with this way that looks what I want:

I don't know what session_id is, is it the same as the session_id of the user created when he logs-in?
Really got tired looking for some straight forward solutions, any help out will be strongly appreciated.